The present invention generally relates to the utilization of an ion-sensitive field-effect transistor and, more particularly, to a device and a method for compensating for variations in characteristics of the ion-sensitive field-effect transducer which may result from changes in temperature.
More specifically, the present invention pertains to an ion activity monitoring device utilizing the ion-sensitive field-effect transistor for the measurement of the pH value of electrolytic fluid present, for example, in a living body and also to a method for using the ion-sensitive field-effect device.
Various types of pH measuring devices utilizing the ion-sensitive field-effect transistor, hereinafter referred to as ISFET have hitherto been well known in the art. The principle of pH measurement with these types of devices is based on the utilization of a change in gate potential of the ISFET device which results from the sensitivity thereof to the activity of H.sup.+ ions contained in the electrolytic fluid of a living body while a constant current or voltage is supplied to the source-to-drain passage of the ISFET device, with the resultant pH value being delivered from the source potential.
In these prior art measuring devices, since the ISFET device tends to be affected not only by the H.sup.+ ions in the electrolitic fluid of a living body, but also by the ambient temperature, an accurate and precise measurement of the ion activity requires compensation for the temperature-dependent changes of the characteristic of the ISFET device.
British Patent Specification No. 2,035,577, published June 18, 1980, which corresponds to U.S. Pat. No. 4,267,504 discloses a device for measuring the quantity, for example, the concentration of ions in a solution such as the electrolytic fluid of the living body which will influence the ISFET device included in the pH measuring device. According to this publication, the compensator disclosed therein is intended for use in the measuring device utilizing the ISFET device as a variable resistance and is so designed that, in order to compensate for temperature-dependent changes of the ISFET device, an auxiliary signal, having a frequency located outside the frequency range of the signal representing the quantity to be measured, can be applied to the transistor, and these two signals are then separated from one another after having been processed by the measuring device. With this compensator, since both the signals of interest, that is, the signal representing the quantity to be measured, and the auxiliary signal applied to the transistor contain a signal component affected by the ambient temperature, the processing of these signals through the measuring circuit theoretically results in an output from the measuring circuit which is free from the affects of the ambient temperature.
While the measuring device disclosed in the above mentioned publication is satisfactory in that any adverse influence brought about by the ambient temperature can substantially be eliminated, it has been found that the signal of interest tends to be inevitably strained considerably at the time the auxiliary signal is applied and also at the time it is separated from the auxiliary signal in the measuring circuit. Therefore, the utilization of the measuring device disclosed in the above mentioned publication brings about a drawback in that an accurate and precise measurement tends to be adversely affected.